A galactic microquasar mimicking winged radio galaxies

Abstract

A subclass of extragalactic radio sources known as winged radio galaxies has puzzled astronomers for many years. The wing features are detected at radio wavelengths as low-surface-brightness radio lobes that are clearly misaligned with respect to the main lobe axis. Different models compete to account for these peculiar structures. Here, we report observational evidence that the parsec-scale radio jets in the Galactic microquasar GRS 1758-258 give rise to a Z-shaped radio emission strongly reminiscent of the X and Z-shaped morphologies found in winged radio galaxies. This is the first time that such extended emission features are observed in a microquasar, providing a new analogy for its extragalactic relatives. From our observations, we can clearly favour the hydrodynamic backflow interpretation against other possible wing formation scenarios. Assuming that physical processes are similar, we can extrapolate this conclusion and suggest that this mechanism could also be at work in many extragalactic cases.

Fig. 1

Z-shaped radio morphology of the microquasar GRS 1758-258. This map was obtained from the concatenation of VLA runs carried out at the 6 cm wavelength in the D and C array configurations conducted in 1992, 1993, 1997 (archival data) and 2016 (new data from this work). Details are given in the Methods section and Table 1. The central core and the north and south radio lobes are labelled together with the unrelated sources #1, #2 and #3 in the same field of view. The white dashed line outlines the Z-shaped appearance of the faint emission extensions emanating from both lobes. The bottom left ellipse shows the angular resolution and corresponds to the full width at half maximum of the VLA synthesised beam (11.31 × 7.18 arcsec², with a position angle of 13°). The right horizontal bar (30 arcsec long) provides the map angular scale, with north being up and east being left. The brightness levels are illustrated by the colour bar to the right. The intensity scale used is logarithmic and expressed in Jansky units (1 Jy = 10⁻²⁶ W m⁻² Hz⁻¹). Meaningful radio emission starts at the 4σ level of approximately 15 μJy

Fig. 2

Comparison of the central core and northern hot spot positions in GRS 1758-258. The equatorial coordinates of these microquasar features during selected epochs of VLA observations are displayed in the J2000.0 reference system. The a and b panels correspond to the central core and the northern hot spot, respectively. Although the central core remains fixed within astrometric errors, the hot spot experiences clear shifts on the order of a few arcsec in time scales of years. Error bars correspond to one standard deviation, as reported by the JMFIT task of AIPS

Fig. 3

Cloud of CO along the GRS 1758-258 line of sight. This map has been created from the Dame et al. survey of CO emission in the Galactic Plane. The cloud is outlined with red contours and its emission peaks at a radial velocity of 210 km s⁻¹ with respect to the local standard of rest (LSR), corresponding to a kinematic distance of 8.5 kpc. The position of the microquasar is marked with a white circle, and the dashed line is coincident with the position angle of the jets. The horizontal bar (15 arcmin long) gives the angular scale. North is up, and east is left. The brightness scale corresponds to the antenna temperature of CO emission in Kelvin

Fig. 4

Slice of CO emission centred on GRS 1758-258. The black squares taken from the Dame survey illustrate how the CO antenna temperature changes along the position angle of the microquasar jets. Error bars are defined as one standard deviation of the emission-free background. The left side corresponds to the direction of molecular cloud impingement, whereas the right side denotes the region far away from it. The dashed line displays the exponential fit used in the main text to estimate the density variations on scales comparable to the jet angular size (3 arcmin  long thick horizontal bar)